Placenta Previa: Practice Essentials, Pathophysiology, Etiology

Placenta previa is an obstetric complication that classically presents as painless vaginal bleeding in the third trimester secondary to an abnormal placentation near or covering the internal cervical os. However, with the technologic advances in ultrasonography, the diagnosis of placenta previa is commonly made earlier in pregnancy. Historically, there have been three defined types of placenta previa: complete, partial, and marginal. More recently, these definitions have been consolidated into two definitions: complete and marginal previa.

A complete previa is defined as complete coverage of the cervical os by the placenta. If the leading edge of the placenta is less than 2 cm from the internal os, but not fully covering, it is considered a marginal previa (see the following image). Because of the inherent risk of hemorrhage, placenta previa may cause serious morbidity and mortality to both the fetus and the mother.

Complete placenta previa noted on ultrasound.

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Another ultrasound image clearly depicting complete placenta previa.

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